This photo provided by DroneBase shows the collapsed pedestrian bridge at Florida International University in the Miami area on Thursday, March 15, 2018. (DroneBase via AP)

By ADRIANA GOMEZ LICONAssociated Press

MIAMI — An innovative pedestrian bridge being built at Florida International University had been put to a “stress test” and its cables were being tightened when it collapsed over traffic, killing six people and sending 10 to a hospital, authorities said.

As state and federal investigators worked to determine how and why the five-day-old span failed on Thursday, one factor may have been the stress test that Miami-Dade Mayor Carlos Gimenez said crews were conducting on the span.

Two workers were on the 950-ton bridge when it pancaked on top of vehicles waiting at a stoplight.

Miam-Dade Fire Rescue personnel work after a brand new, 950-ton pedestrian bridge collapsed in front of Florida International University, Thursday, March 15, 2018, in Miami. Florida officials said Thursday that several people have been found dead in the rubble of the collapsed South Florida pedestrian bridge where the frantic search for any survivors continued past nightfall. (Michael Laughlin/South Florida Sun-Sentinel via AP)

This photo provided by DroneBase shows the collapsed pedestrian bridge at Florida International University in the Miami area on Thursday, March 15, 2018. (DroneBase via AP)

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A Miami-Dade Fire Rescue firefighter takes pictures after a brand new, 950-ton pedestrian bridge collapsed in front of Florida International University, Thursday, March 15, 2018, in Miami. Florida officials said Thursday that several people have been found dead in the rubble of a collapsed South Florida pedestrian bridge where the frantic search for any survivors continued past nightfall. (Michael Laughlin/South Florida Sun-Sentinel via AP)

This March 10, 2018 photo shows an early morning view of the main span of the a pedestrian bridge that is being positioned to connect the City of Sweetwater, Fla., to Florida International University near Miami. The 950- ton new bridge collapsed Thursday, March 15, over several cars causing several fatalities and injuries. (Pedro Portal/ The Miami Herald via AP)

this March 10, 2018 photo shows an early morning view of the main span of the a pedestrian bridge that is being positioned to connect the City of Sweetwater, Fla., to Florida International University near Miami. The 950- ton new bridge collapsed, Thursday, March 15, over several cars causing several fatalities and injuries. (Pedro Portal/ The Miami Herald via AP)

A brand new, 950-ton pedestrian bridge collapsed in front of Florida International University, Thursday, March 15, 2018, in Miami. Florida officials said Thursday that several people have been found dead in the rubble of a collapsed South Florida pedestrian bridge where the frantic search for any survivors continued past nightfall. (Michael Laughlin/South Florida Sun-Sentinel via AP)

A brand new, 950-ton pedestrian bridge collapsed in front of Florida International University, Thursday, March 15, 2018, in Miami. Florida officials said Thursday that several people have been found dead in the rubble of a collapsed South Florida pedestrian bridge where the frantic search for any survivors continued past nightfall. (Michael Laughlin/South Florida Sun-Sentinel via AP)

U.S. Sen. Marco Rubio tweeted late Thursday that the cables that suspend the bridge had loosened and the engineering firm ordered that they be tightened. “They were being tightened when it collapsed,” he said on Twitter.

First responders had been racing to find survivors in the rubble of the 175-foot span using high-tech listening devices, trained sniffing dogs and search cameras before turning the scene over to police.

“This has turned from a rescue to a recovery operation,” Miami-Dade Police Det. Alvaro Zabaleta said.

The $14.2 million pedestrian bridge was supposed to open in 2019 as a safe way to cross the busy six-lane road between the university campus and the community of Sweetwater, where many students live.

At the accident scene, Florida Gov. Rick Scott said investigators will get to the bottom of “why this happened and what happened,” and if anyone did anything wrong, “we will hold them accountable.”

National Transportation Safety Board chairman Robert Sumwalt III said a team of specialists would begin its investigation Friday morning.

Rubio, who is an adjunct professor at the school, noted the pedestrian bridge was intended to be an innovative and “one-of-a-kind engineering design.”

Renderings showed a tall, off-center tower with supporting cables attached to the walkway. When the bridge collapsed, the main tower had not yet been installed, and it was unclear what builders were using as temporary supports.

An accelerated construction method was supposed to reduce risks to workers and pedestrians and minimize traffic disruption, the university said. The school has long been interested in this kind of bridge design; in 2010, it opened an Accelerated Bridge Construction Center to “provide the transportation industry with the tools needed to effectively and economically utilize the principles of ABC to enhance mobility and safety, and produce safe, environmentally friendly, long-lasting bridges.”

The project was a collaboration between Munilla Construction Management, a Miami-based contractor, and Figg Bridge Design, based in Tallahassee. Figg is responsible for the iconic Sunshine Skyway Bridge across Tampa Bay.

Figg’s statement Thursday said the company was “stunned” by the collapse and would cooperate with investigations.

“In our 40-year history, nothing like this has ever happened before,” the statement said. “Our entire team mourns the loss of life and injuries associated with this devastating tragedy, and our prayers go out to all involved.”

MCM Construction Management promised on its Facebook page to participate in “a full investigation to determine exactly what went wrong.”

Robert Bea, a professor of engineering and construction management at the University of California, Berkeley, said it was too early to know exactly what happened, but the decision to use what the bridge builders called an “innovative installation” over a heavily traveled thoroughfare was risky.

“Innovations take a design firm into an area where they don’t have applicable experience, and then we have another unexpected failure on our hands,” Bea said after reviewing the bridge’s design and photos of the collapse.

The FIU community, along with Sweetwater and county officials, held a “bridge watch party” on March 10 when the span was lifted from its temporary supports, rotated 90 degrees and lowered into what was supposed to be its permanent position.

FIU President Mark Rosenberg in a video shared on Twitter Friday that the “tragic accident of the bridge collapse stuns us, saddens us.”

“The bridge was about collaboration, about neighborliness, about doing the right thing,” he said. “But today we are sad and all we can do is promise a very thorough investigation in getting to the bottom of this and mourn those who we have lost.”

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Associated Press contributors include Freida Frisaro, David Fischer and Curt Anderson in Miami; Jason Dearen in Gainesville; Kelli Kennedy in Fort Lauderdale; and Tamara Lush in St. Petersburg.